We are studying the metabolism and enzymes of 2,3-diphosphoglycerate (2,3-DPG) in the human red blood cell. 2,3-diphosphoglycerate modifies the oxygen carrying capacity of hemoglobin. We had shown earlier that bisphosphoglycerate synthase (mutase), the enzyme that catalyzes the synthesis of 2,3-DPG, could also catalyze the hydrolysis of 2,3-DPG. Now it appears that the bisphosphoglycerate phosphatase activity that was previously characterized as a separate enzyme is in fact due to the synthase itself. This also follows from studies of Rosa et al. (Biochem. Biophys. Res. Commun. 51: 536, 1973). The present investigation led to a modification of our earlier rate equation for the bisphosphoglycrate activity of the human red cell. Both phosphoglycerate mutase and bisphosphoglycerate synthase can catalyze the same three phosphoryl transfer reactions, albeit at very different rates. These reactions can be designated as mutase (3-PGA: 2,3-DPG only yields (reversibly) 2-PGA), synthase (1,3-DPG yields 2,3-DPG), and phosphatase (2,3-DPG yields PGA plus Pi). We had shown earlier that both enzymes can be phosphorylated on single histidine residues by glycerate-1,3-P2 or 2,3-DPG. We have now measured the rates of phosphorylation and phosphoryl transfer with a rapid quench apparatus that can measure rates as fast as 20 milliseconds. These studies indicate that the phosphoenzymes have kinetic properties compatible with their participation in the reactions catalyzed by these enzymes.